80mm Dial Flue Gas Thermometer - 150mm stem, 0-500°C

As a rule it is desirable to ensure complete, clean combustion with a maximum percentage of CO2 and at the same time minimum temperature of flue gas. This of course is very difficult and depends on variations in fuel, draft, atmospheric conditions, and mechanical wear.
The ultimate CO2 of oil or gas combustion is only of theoretical value, because it depends on various influences and also domestic standards concerning combustion. Adjusting burner and boiler to maximum combustion efficiency and to meet domestic standards and laws is a difficult business, but possible if percentage of CO2 and net temperature of combustion are known. Consult the manufacturer of heating equipment or fuel supplier for specific recommendations.